I understand your problem, it can be very frustrating teaching multi-level classes. But there are lots of things you can do.

One of the easiest for you is to think carefully about how you group the students. For example, if you are doing a speaking activity you could group the strongest students together. This gives them an opportunity to expand on the topic and the weaker students can work at their speed and level.

If you are doing language work, you could pair the stronger students with the weaker ones. This way the stronger ones can help their peers and possibly take on a role of explaining the language point.

If you are doing reading or listening, you can create different activities for the same text or cassette. This will take you more time but in the end will save frustration and difficulty in the classroom. You can also have lessons where the students are working on different tasks entirely and your role is to monitor and help where necessary.

Just one more point, you could talk to somebody about the placement test that is used to avoid this happening again in the future. Also, try to incorporate activities in the first few lessons that will allow you to check levels while there is still an opportunity to move students.